Development and the Phylogenetic Features of the Middle Ear Region

Overview

Comparing maturing chondrocranium, early ossification, and maturing ossification stages shows that the facial nerve and ganglion, its palatine and chorda tympani branches, the medial and lateral carotid sympathetics, the internal carotid artery, the tensor tympani muscle and nerve, and the tympanic and auricular branches of IX–X nerves are all fairly consistent landmarks with a definable pattern at each stage in the monotremes, marsupials, and placentals.

Using the above soft tissues, the alicochlear commissure; the prootic incisure; incudal, squamosal, and mastoid components of the crista parotica; and the membranous roof and floor of the cavum supracochleare can be placed in a hypothetical morphotype of early stages of all of the taxa examined.

It is much more problematic to trace this into the pattern of osteological features that are used as diagnostic features in phylogenetic studies. Skeletal development, in both cartilage and bone, is extremely dynamic in later stages. At many sites, different bone or cartilage centers provide essentially similar components that are rationally scored as different because they are parts of different bones. Simple binary logic becomes risky when applied to such components.

The chondrocranial morphotype should provide for an alicochlear commissure, a marked cavum supracochleare, a marked enclosure of the sulcus facialis by the crista parotica, a strong development of the crista supporting incus, a tympanohyal bridge, a chordafortsatz, a prootic venous incisure, and a stapedial blastema pierced by the stapedial artery. The pyriform fenestra may be apomorphic, but which, if any, parts of the roof were from squamosal, petrosal, alisphenoid, etc., and which passages were present or separate can be expected to be variable in closely related forms near to the last common ancestor.